The invention relates to an apparatus which is secured in a rectangular parallelepipedal metal mounting shell, the mounting shell being inserted into an opening in a mounting wall and secured to the mounting wall by means of bent tongues. At least one tongue is formed in each of four walls of the shell. The tongues lie in planes perpendicular to the plane of the mounting wall, and are each formed by a cut which leaves a bridge portion between the relevant tongue and the shell. The mounting shell includes an abutment which bears against at least a portion of the outer surface of the mounting wall surrounding the opening with a given force which is produced by the bending of the tongues. The apparatus is then held by fastening means on the apparatus and on the shell which engage one another when the apparatus is inserted into the shell.
In an apparatus of this kind described in German patent application No. 2,903,176, the bridge portion which connects the tongue to the remainder of the mounting shell is situated near the abutment which is then formed by a flange or upright edge. The distance between the bridge portion and the flange or upright edge is dependent on the thickness of the mounting wall. This means that even though excellent results are obtained in the case of comparatively thick mounting walls, deformation of the portion of the mounting shell which is situated between the bridge portion and the extreme edge of the mounting shell can occur in the case of comparatively thin mounting walls.